The present invention relates to body implantable devices, and more particularly to prostheses and grafts intended for long-term or permanent fixation in body cavities.
A wide variety of patient treatment and diagnostic procedures involve the use of devices inserted into the body of the patient, with some of these devices being permanently implanted. Among these devices are prostheses or grafts for transluminal implantation, for example as disclosed in U.S. Pat. No. 4,655,771 (Wallsten). The prosthesis described in Wallsten is a flexible tubular braided structure formed of helically wound thread elements. Gripping members at opposite ends of the prosthesis initially secure it to a catheter, with the proximal gripping member being movable distally to give the prosthesis the shape of a balloon. In deployment, the gripping members and catheter are removed, leaving the prosthesis to assume a substantially cylindrical shape as it slightly expands and substantially conforms to a blood vessel wall or other tissue. Another prosthesis is disclosed in U.S. Pat. No. 4,681,110 (Wiktor). A flexible tubular liner, constructed of braided strands of a flexible plastic, is insertable into the aorta, whereupon it self-expands against an aneurysm to direct blood flow past the aneurysm. The braided stents of Wallsten and Wiktor axially contract as they radially expand.
Another elastic stent is shown in U.S. Pat. No. 4,830,003 (Wolff et al). The stent includes a series of generally longitudinal wires welded together in pairs, with the wires in each pair then bent into a "V" shape. Like the braided stents, this stent shortens axially as it radially expands.
Prostheses also have been constructed of plastically deformable materials. U.S. Pat. No. 4,733,665 (Palmaz) discloses intraluminal vascular grafts radially expanded using angioplasty balloons. The grafts are wire mesh tubes, and axially shorten as they radially expand. U.S. Pat. No. 4,800,882 (Gianturco) features a stent formed of wire, including a plurality of serpentine bends to form opposed loops. A balloon is inflated to radially expand the stent, without substantial axial shortening.
Yet another approach to prosthesis design is shown in U.S. Pat. No. 3,868,956 (Alfidi et al). Alfidi et al discloses a strainer or screen with a plurality of generally longitudinal wires, bound together by a cylindrical sleeve. The wires are deformable into a longitudinal, straight-line configuration for implantation. Once implanted, the device is heated. Due to the recovery property of the metal forming the wires (e.g. nitinol alloy), heating causes the wires to flare radially outward at the opposite ends, thus to secure the device at the desired location.
A stent including means for maintaining a constant axial length in spite of radial expansion or contraction, is disclosed in U.S. Pat. No. 4,553,545 (Maass et al), as a prosthesis in the form of a helical coil spring. In one embodiment, a constant axial length of the spring is maintained, with opposite ends of the spring rotated relative to one another to change the spring pitch and radius. An alternative approach involves maintaining a constant pitch over a given section of a spring, by providing spring material to a "constant length" section from a more compressed section of the spring. In each case, the spring preferably is elastic, with a memory favoring the radially expanded configuration.
A self-expanding stent or prosthesis often is preferred over a plastically deformed device. Resilient stents can be deployed without dilatation balloons or other stent expanding means. A self-expanding stent can be preselected in accordance with the diameter of the blood vessel or other fixation site. While deployment requires skill in positioning the prosthesis, the added skill of properly dilating the balloon to plastically expand a prosthesis to a selected diameter is not required. Also, the self-expanding device remains at least slightly compressed after fixation, and thus has a restoring force which facilitates acute fixation. By contrast, the plastically expanded stent must rely on the restoring force of deformed tissue, or on hooks, barbs or other independent fixation means.
Further advantages arise from constructing the prosthesis of multiple, braided and helically wound strands or filaments as in the aforementioned Wallsten patent. The filaments themselves have a restoring force which causes the filaments to bear against tissue walls of the body cavity in which the stent is fixed, thus maintaining the cavity open. At the same time there is sufficient space between adjacent filaments to promote embedding of the stent into the tissue, and fibrotic growth to enhance long-term fixation. A further advantage of this construction is that it enables a substantial radial contraction of the prosthesis during deployment, for example to as little as about one-fourth of the normal diameter (the diameter in the relaxed state, i.e. when subject to no external forces). This facilitates deployment of the prosthesis through narrow vessels or other constrictions on the way to the point of fixation.
At the same time, a substantial axial elongation accompanies the radial contraction. There is a substantial axial contraction or shortening as the stent self expands, once free of its radial constraint. Thus, there is a rubbing or scraping action axially along tissue as the radially expanding stent also axially shortens. Should tissue at the fixation area further yield to radial prosthesis expansion in the longer term, such expansion causes further axial shortening and wiping action, and presents further risk of injury to tissue. A further drawback is that a stent during its fixation may radially expand more than expected, retaining less than the intended or minimum necessary axial length. Likewise, a plastically deformable stent may require more than the anticipated radial expansion and axial shortening.
Therefore, it is an object of the present invention to provide a prosthesis of open weave, helical and braided construction capable of substantially maintaining its axial length as it radially self-expands.
Another object is to provide a radially expanding tubular stent comprised of at least two stent segments, with an area of overlap of the sections variable in axial length to maintain a consistent axial separation between non-overlapping ends of the stent.
Yet another object is to provide a stent with a medial portion variable in axial length, in combination with means at the opposite end portions of the stent for fixing the stent to bodily tissue, such that the bodily tissue maintains a substantially constant axial separation of the two end portions during any radial expansion or contraction of the stent.